1. Field of the Invention
The present invention relates generally to a method of detecting a variation in stray capacitance and, more particularly, to a method of accurately detecting and finding out a variation in the stray capacitance in a wide range from an infinitesimal fluctuation to a relatively large fluctuation by use of a phase-locked loop circuit (hereinafter referred to as a PLL circuit) that could not be obtained by a conventional detecting method.
2. Related Background Art
It is required to detect a variation in stray capacitance more accurately and simply for foreseeing a danger and a notifying operation in a place where the stray capacitance varies concomitantly with a change or fluctuation in state or situation of a body to be detected as in the case of: detecting and finding out approaching and contact states or a release state in such portions that an approach to and contact with or a release from, e.g., a human body and a solid-state substance are not permitted; uncovering an intermix of air bubbles and foreign matters in an interior of a liquid material transfer pipe such as a hydraulic brake hose in which an existence of the air bubbles is not allowed; and further monitoring a level of storage quantity of the liquid material in a liquid material storage tank, etc.
A variety of detecting apparatuses have hitherto been known as the above-mentioned apparatus for detecting the variation in the stray capacitance. For example, there is an apparatus for detecting the variation in the stray capacitance of a sensor that is caused corresponding to a fluctuation in state of the detected body. This apparatus is, as illustrated in a functional block diagram of FIG. 12, constructed of an oscillator 1, a wave detection circuit 2, a filter 3, a voltage comparing circuit 4 and an output circuit 5. Note that in FIG. 12, 1a designates an oscillation circuit thereof; 1b a frequency determining block; 6 a sensor; 7 an alarm; and 8 a reference voltage. The oscillator 1 to which the sensor 6 is connected consists of the frequency determining block 1b having a constant quality factor Q and the oscillation circuit 1a. Further, a frequency of the oscillator 1 can be set high or low. If set high, the frequency determining block 1b of the oscillator 1 is composed mainly of a coil and a capacitor.
FIG. 13 is a functional block diagram showing a constructive example of the oscillator 1 in the conventional apparatus, illustrated in FIG. 12, for detecting the variation in the stray capacitance. The same symbols in FIG. 13 as those in FIG. 12 indicate the like functional blocks in FIG. 12. However, the symbol L represents a coil, and C denotes a capacitor. Now, when setting a frequency as high as 1 MHz-200 MHz to the above-mentioned oscillator 1, the frequency determining block 1b is composed chiefly of the coil L and the capacitor C that are connected in parallel to have a constant resonant frequency and the quality factor Q.
Referring to FIG. 12, if the connected sensor 6 is made to approach or contact the detected body, the wave detection circuit 2 is informed of fluctuations in oscillation output amplitude voltage and in output frequency of the oscillator 1 that are caused with changes both in the resonant frequency and in the quality factor Q of this frequency determining block 1b or a stoppage of an oscillating state. The wave detection circuit 2 detects an inputted signal and inputs a voltage level which has been converted into a DC component to the filter 3 of the next stage. The filter 3 having a fixed time constant smooths the DC component by making it pass therethrough. A DC-converted voltage is compared with the preset reference voltage 8 in the voltage comparing circuit 4. This compared result is detected as a variation in the stray capacitance. This detected result is transferred via the output circuit 5 to the alarm 7. The variation in the stray capacitance that is detected by the sensor 6 is notified as a piece of information indicating that there is a change in state of the detected body. Note that some of the apparatuses for detecting the variation in the stray capacitance at a relatively high accuracy are constructed to detect a fluctuation in the amplitude voltage more clearly by connecting a tuning circuit (or band-pass filter) to the output side of the oscillation circuit 1a of FIG. 12.
When using the oscillator 1 having a high frequency, however, this effectively acts to an infinitesimal fluctuation sensed by the sensor 6. While on the other hand, however, the sensor 6 itself turns out to be an antenna, resulting in a state where the electric waves are constantly radiated. This conduces to a possibility in which a hindrance to the electric waves arises in ambient areas. Hence, it is impossible to take a relatively large distance between the sensor 6 and the oscillator 1. On the other hand, if the distance between the sensor 6 and the oscillator 1 is taken large by employing a coaxial cable or the like, a sensitivity to the infinitesimal change in the stray capacitance declines due to a self-capacitance possessed by the coaxial cable itself. In some cases, the oscillating state can not be maintained, and the detection range of the stray capacitance is therefore remarkably narrowed. Further, problems arise, wherein even if the sensitivity of the oscillator 1 is enhanced including the sensor 6, a sharpness is provided, and therefore the operation becomes unstable, or a malfunction is induced. Namely, the conventional detecting apparatus merely continues the oscillation stopping condition against the large variation in the stray capacitance. The actually operating range is limited so much. As described above, the conventional method of setting the oscillator 1 at the high frequency, though it exhibits an increased sensitivity in the very limited range, presents a good number of disadvantages wherein the type of the detected body is limited and so on because of causing the electric wave hindrance and the impossibility to take a large distance between the apparatus body and the sensor 6.
In view of these points, there is also a prior art apparatus of such a system that the oscillator 1 is set at a relatively low frequency on the order of, e.g., 500 KHz. In the case of this system, the frequency determining block 1b serving as a main component is constructed of the capacitor and the resistor. FIG. 14 is a functional block diagram illustrating a constructive example in the case of setting the oscillator 1 at a relatively low frequency. The same symbols in FIG. 14 as those in FIG. 12 indicate the identical functional blocks with those in FIGS. 12 and 13. However, the symbol R designates the resistor. In the oscillator 1 in FIG. 14, the frequency determining block 1a is composed of the resistor R and the capacitor C that are basically connected in series.
In the apparatus, of FIG. 12, constructed to set the oscillator 1 at the low frequency, the capacitance variation detected by the sensor 6--i.e., a frequency fluctuation produced during a detection of the stray capacitance and a fluctuation in the oscillation output amplitude voltage--are replaced with fluctuations in the voltage level of the DC component by means of the wave detection circuit 2 and the filter 3 as well. The voltage comparing circuit 4 compares an obtained voltage with the preset reference voltage 8, thereby detecting a variation in the stray capacitance. This point is the same as that in the case of setting the oscillator 1 at the high frequency.
The detecting system which uses the low frequency has the following disadvantages. The sensitivity to the variation in the stray capacitance is low because of the low frequency. However, the fluctuations both in the frequency and in the oscillation output amplitude voltage are produced infinitesimally corresponding to the variation in the stray capacitance. However, if the fluctuations are converted into voltage by means of a circuit connected to a posterior portion, since the fluctuation quantity is too small, the comparison is quite difficult. It is because a difference from the preset reference voltage 8 is infinitesimal. This lacks the practicability in combination with the low sensitivity given above.
As explained above, the foregoing problems are inherent in the conventional apparatuses for detecting the variation in the stray capacitance in any system, whether the oscillator frequency is set high or low.